Expose model upgrade retirement times (#38449)

## What changed

- Parse the optional `retirement_at` RFC 3339 value from model upgrade metadata, treating missing, null, or malformed values as unknown.
- Expose known retirement times from `model/list` as nullable Unix timestamps in `upgradeInfo.retirementAt` and preserve them when converting app-server models back into model presets.
- Update the generated protocol schemas and app-server documentation.

## Testing

- Cover absent, null, valid, and malformed catalog values.
- Verify `model/list` serialization and app-server model conversion for known, missing, and out-of-range timestamps.

GitOrigin-RevId: 969baf9f5b012997f460fb4611ba63c963ee658c

codex-rs/app-server-protocol/schema/json/v2/ModelListResponse.json

@@ -197,6 +197,14 @@
             "null"
           ]
         },
+        "retirementAt": {
+          "description": "Informational Unix timestamp for this upgrade's scheduled retirement, if known.",
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
         "upgradeCopy": {
           "type": [
             "string",